Investigators seeking early answers for the horrific bomb blasts that scarred the Boston Marathon were no closer to revealing any progress Tuesday.

All they confirmed is a pressure cooker stuffed with gunpowder and shrapnel caused at least one of the blasts that killed three people and injured 176 others in the worst attack on U.S. soil since Sept. 11, 2001.

And investigators who found pieces of black nylon at the scene suspect the bombs used in Mondays attack were in dark-colored bags that would have been heavy to carry, according to FBI Special Agent in Charge Richard DesLauriers.

Earlier, hopes that a quick breakthrough in the investigation had been reached ended with law enforcement sources saying that a Saudi Arabian student injured in the blast was likely to be cleared of suspicion.

The injured and the maimed are now being treated at half a dozen hospitals in the Boston area. Young spectators especially paid a heavy price.

Trauma surgeons said at press briefings that a number of victims had a range of metallic shrapnel removed during surgery, including pellets and what appeared to be carpenter nails.

"The vast majority of the injuries were to lower extremities," said Dr. Tracey Dechert, a trauma surgeon at Boston Medical Center, which treated 23 people and performed amputations on five of them.

"When these kids came in ... they were just so badly hurt, just covered with singed hair and in so much pain, it was just gut-wrenching," added David Mooney, the director of the trauma program at Boston Children's Hospital. "Pulling nails out of a little girl's flesh is just awful."

At least 10 people had limbs amputated as a result of their injuries.

The youngest to die in the attacks was an eight-year-old boy.

In a statement, his family identified him as Martin Richard, who lived in the city's Dorchester neighbourhood.

Outside the family's home, sympathizers created a makeshift memorial of flowers and "Peace" was written in chalk on the sidewalk.

Officials identified a second person killed as Krystle Campbell, 29, of Medford, Mass.

The third fatal victim of Monday’s Boston Marathon bombings was a Chinese citizen whose identity was not being made public at the request of the victim’s family, the Chinese Consulate in New York said in a statement on Tuesday.

Another Chinese citizen was injured and in stable condition after surgery, the statement said.

As flags flew at half-mast across the country, President Barack Obama, who will travel to Boston on Thursday for a memorial service, called the two bombings an "act of terror" and said every effort would be made to find and punish the perpetrators.

He praised the "exhausted runners who kept running to the nearest hospital to give blood, and those who stayed to tend to the wounded, some tearing off their own clothes to make tourniquets.

In an incident at Boston Logan International Airport, two passengers and their bags were removed from a United Airlines flight before departure, a source with direct knowledge of the action told Reuters.

The identities of the two passengers were not disclosed.

Meanwhile, a nearly 1.6-km stretch of Boylston St., near the marathon finish and Boston's two tallest buildings, remained cordoned off Tuesday evening and a 12-block portion of that street could remain closed for several days, police said.

Boston Police Commissioner Ed Davis told reporters that no arrests had been made.

Davis called it "the most complex crime scene that we have dealt with in the history of our department."

While federal authorities are saying that there appears to be no further threat, there remains a heavy police outside Brigham and Women's Hospital in Boston.

-- with files from Reuters

Team Sun is in Boston to bring you the latest on the bombings. Follow updates from Joe Warmington, Chris Doucette and Simon Kent.

Investigators search for evidence on the rooftop of a building located above the site of a bomb blast on Boylston Street two-days after multiple explosions at the Boston Marathon killed three and injured 176 in Boston, Massachusetts April 17, 2013. Investigators believe they have identified a suspect in the Boston marathon bombing from security video, a U.S. law enforcement source said. REUTERS/Adrees Latif